This has to be the industry standard noise gate. It has two channels that can be linked to work in stereo and controls for threshold, attack, hold and decay. A range control sets how much sound gets through the closed gate and there are adjustable HF and LF filters so that you can focus on the sound that you want to open the gate.As well as gating there is also a ducking mode.

Good Points
This is a brilliantly functional piece of equipment. It's very easy to set up and there is as much control as you could possibly need in when the gate opens and letting just the right amount of sound through it. It works great on drums and can really clean up the sound of a kit.
The filters work well in isolating the area of a sound that will open the gate, but if you don't need to use the 201 as a gate you can put it in key listen mode and use the filters as an EQ to cut the top or bottom out of sounds.
It's also excellent at 'chopping up' sounds. Just feed any sustained sound through the gate and feed a rhythmic pulse, say from a drum machine, into the key input. Set it to external key source, tweak the threshold and the envelope controls and you can get a cool tremolo or the sound of an old analogue sequencer.